Aircraft Shock Strut Having a Fluid Level Monitor

ABSTRACT

A system for monitoring a liquid level in an aircraft shock strut includes a cylinder having an internal chamber sealed by a piston telescopically movable within the cylinder. The chamber contains a gas and a liquid, and a sensor assembly is provided for monitoring a condition of a level of the liquid in the chamber. The sensor assembly includes at least one probe within the chamber, and a fitting assembly allows one or more leads from the probe to pass through the wall of the strut while maintaining pressure in the chamber. The fitting assembly includes a plug molded to the one or more leads extending from the probe. The fitting assembly also includes a retainer for holding the plug in sealed relationship with a through passage in the strut.

FIELD OF THE INVENTION

The present invention relates to aircraft shock struts for absorbing anddamping shock forces, such as during landing, taxiing or takeoff, andparticularly to an “air-over-oil” shock strut.

BACKGROUND OF THE INVENTION

Shock absorbing devices are used in a wide variety of vehicle suspensionsystems for controlling motion of the vehicle and its tires with respectto the ground and for reducing transmission of transient forces from theground to the vehicle. Shock absorbing struts are a common and necessarycomponent in most aircraft landing gear assemblies. The shock strutsused in the landing gear of aircraft generally are subject to moredemanding performance requirements than most it not all ground vehicleshock absorbers. In particular, shock struts must control motion of thelanding gear, and absorb and damp loads imposed on the gear duringlanding, taxiing and takeoff.

A shock strut generally accomplishes these functions by compressing afluid within a sealed chamber formed by hollow telescoping cylinders.The fluid generally includes both a gas and a liquid, such as hydraulicfluid or oil. One type of shock strut generally utilizes an“air-over-oil” arrangement wherein a trapped volume of gas is compressedas the shock strut is axially compressed, and a volume of oil is meteredthrough an orifice. The gas acts as an energy storage device, such as aspring, so that upon termination of a compressing force the shock strutreturns to its original length. Shock struts also dissipate energy bypassing the oil through the orifice so that as the shock absorber iscompressed or extended, its rate of motion is limited by the dampingaction from the interaction of the orifice and the oil.

Over time the gas and/or oil may leak from the telescoping cylinders andcause a change in the performance characteristics of the strut.Presently, there is no reliable method of verifying the correctservicing parameters of aircraft shock struts. While gas pressure can bereadily monitored, it cannot be readily determined if a loss in gaspressure arose from leakage of gas alone or from leakage of both gas andoil, unless external evidence of an oil leak is noticed by maintenancepersonnel. If a low pressure condition is detected in the absence ofexternal evidence of an oil leak, maintenance personnel heretofore wouldrestore the gas pressure to a prescribed level by adding gas. This,however, eventually leads to degraded performance of the shock strut oilhad indeed escaped from the strut. Even if evidence of a oil leak isobserved, maintenance personnel cannot easily determine how much oilremains or whether the remaining amount of oil meets specifications oris acceptable for operation.

Two methods can be used to determine whether a strut has the correctpneumatic charge. One method is to jack-up the aircraft to take theweight off of the struts such that each strut is fully extended. Theproper pressure that corresponds to the extended position of the strutis a known value. In the other method the pressure is measured with theaircraft supported by the strut using a pressure gauge, and the strokeis measured to determine the extension of the strut. Variations in theweight of the aircraft and the position of the center of gravity causethe strut to sit at a variety of strokes in this situation. A look-uptable or chart is then used to verify that the stroke and the pressurematch an acceptable value. Since jacking the aircraft is rarely done andis very time consuming, the method of verifying the pressure with theaircraft supported by the strut in a static position is most commonlyused. This latter technique, however, is not a very reliable way tocheck the oil level.

The only reliable way to know that the oil level is acceptable is tovent the pneumatic charge and pump oil through the strut to ensure aproper oil level. The strut can then be re-inflated with gas to theproper pressure. This operation takes a significant amount of time, andas a result maintenance personnel may skip this step and only correctthe pressure by adding or venting gas. In addition, neither techniqueenables detection of the oil level while the aircraft is in flight.

SUMMARY OF THE INVENTION

The present invention provides a shock strut that includes a probe fordetecting a condition of a liquid level in the strut. Thus maintenancepersonnel, or perhaps even a flight crew, can readily ascertain whetherthe liquid level in the strut is within acceptable parameters and canmonitor the liquid level for leaks. In addition, the present inventionprovides a sensor assembly having such a probe that can be removed fromthe strut as a unit, thereby facilitating repair and maintenance of thesensor assembly.

In particular, the, present invention provides an aircraft shock strutthat includes a cylinder, and a piston telescopically movable within thecylinder. The piston and the cylinder define a sealed chamber that ispartially filled with a liquid and partially filled with a gas. Thestrut also includes at least one probe associated with the chamber forsensing a condition of a level of liquid in the chamber.

The strut may be part of a system that further includes a processor incommunication with the probe for processing a signal from the proberelated to the level of liquid in the chamber. The strut may include acable that passes through a wall of the strut for connecting to theprobe, such as an optical fiber cable. A fitting assembly engages thecable and seals a passage through the strut wall to the chamber. Thefitting assembly preferably includes a plug molded around the cable anda retainer for holding the plug in sealed relationship with a throughpassage in the strut. The cable may include at least one optical fiberor a plurality of optical fibers that have transversely spaced apart,coextending portions, each of which is surrounded in sealed relationshipby the plug that has been molded thereto. The shock strut may includetwo probes, a first one of which detects a first condition of the liquidlevel and a second one of which detects a second condition of the liquidlevel.

The probe and cable may be assembled together as a unit that isremovable as a unitary piece from the strut. The strut may also includea guide tube mounted within the chamber so that the unit at leastpartially extends through and is located by the guide tube.

The present invention also provides a method of monitoring a conditionof a liquid level in an aircraft shock strut. The method includes thesteps of receiving a signal related to the liquid level from at leastone probe of a shock strut, and processing the signal to determine acharacteristic of the liquid level in the strut.

The monitoring system provided by the present invention also may allowpersonnel removed from the landing gear and the strut, such as a pilot,to check a condition of the liquid level, such as whether the level ofliquid is below a specified minimum in the strut, either after takeoffand before the landing gear is retracted or after extending the strutfor landing but before the aircraft touches down on the runway. At thatpoint the strut is not under load and is fully extended, and reliablereadings can be taken that will indicate whether the liquid level in thestrut is acceptable. In addition or alternatively, the data may bestored for later retrieval by maintenance personnel.

The foregoing and other features of the invention are hereinafter fullydescribed and particularly pointed out in the claims, the followingdescription and annexed drawings setting forth in detail a certainillustrative embodiment of the invention, this embodiment beingindicative, however, of but one of the various ways in which theprinciples of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevation view of a simplified landing gearassembly incorporating a shock strut in accordance with the presentinvention.

FIG. 2 is a partial cross-sectional view of a shock strut provided bythe present invention.

FIG. 3 is an enlarged cross-sectional view of an upper end of the shockstrut of FIG. 2 in area A.

FIG. 4 is an enlarged cross-sectional view of a fitting in the shockstrut of FIG. 3 in area B.

FIG. 5 is a schematic illustration of the high and low probes withassociated sensitivity ranges.

FIG. 6 is a further illustration of fluid level sensor indications.

FIGS. 7-12 show another implementation of a shock strut with leveldetection.

DETAILED DESCRIPTION

FIG. 1 presents a simplified aircraft shock strut 10, shown mounted atan upper end to an aircraft structure 20 by an attachment member 22. Anyof a number of types of struts (or shock absorbers) may be used inaccordance with the present invention, provided that they involve aninternal working fluid. A wheel assembly 24 is attached to a lower endof the shock strut 10. The aircraft structure 20, attachment member 22,and wheel assembly 24 are shown in simple or outline form, while otherstructures such as locking mechanisms and retracting mechanisms are notshown in FIG. 1 in order to avoid obscuring the shock strut. Variousarrangements of such structures are known in the art and are notcritical to the description or understanding of the invention.

The shock strut 10 includes a piston 30 and a cylinder 32 which may becylindrical as is customary, or some other shape if desired. Thecylinder and piston respectively have one or more mounting tabs (eyes)33 and 34 for attachment to the aircraft structure 20 and the wheelassembly 24. The piston 30 communicates forces to and from the wheelassembly 24. The cylinder 32 receives the piston 30 in a manner thatpermits relative telescoping movement between the cylinder 32 and thepiston 30 to absorb and dampen shock forces from being transmitted tothe aircraft structure 20.

In accordance with the present invention, a sensor assembly or detector35 is provided for sensing or detecting a condition of a level of aliquid contained within the strut 10 (as in the manner hereinafterdescribed) and outputting a signal related to the liquid level. In theillustrated embodiment, the detector 35 includes a probe assembly 37extending into the strut and a transmitter/receiver or sensor unit 38located outside the strut for transmitting/receiving signals to/from theprobe assembly. The sensor unit 38 communicates the signal from theprobe assembly 37 to a computer, processor or other logic device 39 foranalyzing the signal and for determining a condition of the amount ofliquid in the strut 10.

The processor 39 may have a memory and software associated therewith tocarry out monitoring operations, and may be located near the strut 10,may be remote from the strut or may be a combination with someprocessing and/or data storage occurring near the strut and otherprocessing and/or data storage occurring at another location. Theprocessor also may be coupled to a display (not shown), which may be onthe flight deck or in the wheel well, or may be connected by maintenancepersonnel as needed. The processor 39 can communicate with the sensorunit 38 via an electric or optical cable 40, or by other means such as aradio frequency transmitter and receiver, or other device.

While the sensor unit 38 may be connected to the processor 39 viaelectrical means, preferably the probe assembly 37 is an optical devicethat communicates with the sensor unit 38 via an optical cable 41. Thisadvantageously avoids introduction of electrical signals into theinterior of the strut. Although a capacitance sensor can be used todetermine how much liquid remains in the strut 10, generally this is notdesirable.

Referring now to FIG. 2, the piston 30 and the cylinder 32 of the shockstrut 10 define a sealed elongate chamber 42 at least partially filledwith a liquid, such as hydraulic fluid or oil. The liquid preferably isfilled to a nominal or full level, indicated by the line 44. A portionof the chamber 42 is filled with a gas, such as nitrogen, as is commonin an air-over-oil type of shock strut. The gas generally is pressurizedand the chamber 42 is sealed to provide a pressurized environment insidethe chamber relative to the atmosphere outside the strut 10. Varioussealing arrangements that allow telescoping movement between the piston30 and the cylinder 32 while maintaining a seal are well known.

The illustrated shock strut 10 also includes an orifice plate 50 withinthe chamber 42 that has an orifice opening 52 passing therethrough. Theorifice plate divides the chamber 42 into a pneumatic chamber 54 thathas a gas in the upper portion thereof and a dynamic liquid chamber 58that is partially defined by the piston 30 and which generally is filledwith a liquid. The latter chamber may be referred to as the dynamic oilchamber 56, since oil often is used as the liquid.

A metering tube 58 may be mounted in the pneumatic chamber 54 between anend wall of the cylinder 32 and the orifice plate 50. A metering pin 60is mounted to the end of the piston 30 and extends into the orificeopening 52 as the piston 32 is telescopically compressed into thecylinder 30. The metering tube 58 guides the metering pin 60 andgenerally keeps the pin centered in the orifice opening 52. The meteringpin 60 regulates the size of the area through which the liquid can flow,thereby increasing the resistance to further compression and regulatingthe extension of the piston 30 when the compression force is removed orreduced.

In FIGS. 2 and 3, details of an exemplary detector 35 are shown. Theprobe assembly 37 includes at least one probe 80/82, and the cable 41for connecting the probe or probes to the sensor unit 38. The cable issealed by a pressure fitting assembly 72 to the wall of the strut 10 atan opening in the wall through which the probe assembly 37 passes. Aguide tube 74 is mounted by suitable means within the chamber 42, andpreferably extends from the interior or pressurized end of the pressurefitting assembly 72. The pressure fitting and guide tube are discussedbelow in greater detail.

In the illustrated embodiment, the probe assembly 37 includes two probes80 and 82. The probe 80 functions to detect the presence or absence ofliquid at first location, indicated at 84. The second probe 82 functionsto detect the presence or absence of liquid at a second location,indicated at 86, which represents a lower liquid level that may beoutside the range of specified acceptable liquid levels. For example,when liquid is no longer detected at the first liquid level 84, anindication may be given that the strut should be serviced, perhapswithin the month. When liquid is no longer detected at the second, lowerliquid level 86, an indication may be given that the strut needs servicesooner, perhaps right away before the next flight or within the week.Such detection can be effected by sensing either the presence or absenceof liquid at a given level, depending on the type of probe and itsorientation relative to the fluid level in the strut.

As above mentioned, the probes 80 and 82 preferably are fiber opticprobes. The sensing end or tip of each probe includes a retro-reflectoror retro-reflective corner cube prism exposed to the fluid in thepneumatic chamber 54. When the tip of the probe is covered by liquid,the amount of light reflected is minimal or zero because the relativeindices of refraction of the liquid and the tip of the probe aresimilar. However, in the absence of liquid, a light signal transmittedfrom the sensor unit 38 to the probe is substantially completelyreflected, thereby indicating the absence of liquid at the tip of theprobe.

An exemplary sensor unit 38 is the Mini Beam™ system from BannerEngineering Corporation of Minneapolis, Minn., USA. The sensor unit 38interfaces electrically or electronically with the processor 39. Thesensor unit 38 also acts as a transmitter/receiver that sends andreceives optical or light signals to and from the fiber optic probeassembly 37 along respective optical leads or fibers 73.

The probes 80 and 82 are connected to the sensing unit 38 by the cable41 which includes optical fibers 73 for channeling light from thesensing unit 38 to the probes 80 and 82 and reflected light from theprobes 80 and 82 to the sensing unit 38. In the illustrated embodiment,the optical fibers 73 are continuous, but it will be appreciated thateach fiber may be composed of two or more segments that connect thesensing unit 38 to the probes 80 and 82.

In the illustrated embodiment, proper positioning of the probes 80 and82 within the chamber 42 is effected by the guide tube 74. The guidetube 74 also helps keep the probes 80 and 82 and the optical fibers 73within the chamber 42 away from moving parts of the strut 10. However,other means for positioning and restraining the probes 80 and 82 couldbe used in place of the guide tube 74. The guide tube 74 alsofacilitates installation and/or replacement of the probe assembly 35 aswill be explained in a subsequent paragraph.

The lower end of the guide tube 74 preferably is submerged in the liquidin the chamber 42 and has a bottom opening and/or several lateralopenings 83 communicating with upper portion of the chamber 42, therebyto allow the liquid to pass into and out of the guide tube 74 forcontacting the probes 80 and 82 allowing the level in the guide tube toassume the same level as the balance of the liquid in the balance of thechamber 42. The guide tube 74 has a diameter which is much less than thediameter of the chamber 42, and thus the liquid in the guide tube issubject to less wave action and reduced amplitude wave action inresponse to shock and vibration forces applied to the shock strut 10.However, liquid sensing usually will take place when the aircraft is inflight and the landing gear are extended, at which time wave action willbe minimal. An exemplary guide tube 74 has a diameter of aboutthree-quarters of an inch (about 1.9 cm) and has a length of abouteighteen inches (about 45.7 cm).

The guide tube 74 has one or more spring tabs 90 that hold the guidetube 74 in place in the chamber 42, although other means for securingthe guide tube could be used, including a welded attachment, bolts orrivets. In the illustrated embodiment, several spring tabs 90 help bracethe guide tube 74 between the wall of the cylinder 32 and the meteringtube 58. As seen in FIGS. 3 and 4, the upper end of the guide tube 74 isflared for closely mating with a tapered end of a tubular fitting 94forming part of the fitting assembly 72.

The illustrated pressure fitting assembly 72 includes the tubularfitting 94 that is mounted, as by threading, in a threaded hole 92 inthe wall of the cylinder 32. A packing (not shown) or other suitablesealing means may be interposed between the fitting 94 and the sides ofthe hole 92 to provide a further seal.

The tubular fitting 94 has a through passage 96 in which a plug 98 isreceived and sealed by an O-ring 106 or other suitable means. Inparticular, the plug 98 has an annular groove 104 for receiving theO-ring 106 for providing a sealed interface between the plug 98 and thefitting 94. The through passage 96 and plug 98 preferably arecylindrical. The plug 98 is located between a shoulder 108 formed by areduced diameter end portion of the through passage 96 and a retainer100 that is screwed into or otherwise removably fastened to the fitting94 for retaining the plug 98 in the fitting. Both the fitting 94 and theretainer 100 may be provided with hexagonal head portions to facilitateinstallation. Also, an annular seal and/or washer 110 may be interposedbetween a head flange of the retainer 100 and outer end surface of thefitting 94, as shown. In an alternative arrangement, the fitting 94could be formed as an integral part of the cylinder 32, in which case itobviously would not be removable.

The plug 98 preferably is molded to the cable 41. More particularly, theplug is molded directly to the optical fibers 73 passing therethrough.The optical fibers are transversely spaced from one another so that eachfiber is surrounded by the plug material molded thereabout. Thisarrangement provides an effective seal between the plug 98 and eachfiber 73 without significantly degrading the operation (preferably nogreater than 10% light transmission degradation) of the optical fibers.Although less desirable, the plug 98 could be molded directly to asheath on the optical fiber.

Externally of the fitting assembly 72, the optical fibers are protectedby a sheath 112. In FIG, 4, the sheath 112 is shown broken away, butpreferably extends into a through bore 114 in the retainer 100 throughwhich the optical fibers 73 pass. The optical fibers 73 extend from thefitting 94 into the guide tube 74 and terminate at the probes 80 and 82for conveying signals to and from the sensor unit 38 (FIG. 2) as abovedescribed. The fibers 73 can be individually sheathed, or as shown thefibers associated with a respective probe can pass through a protectivetube 116.

As will be appreciated, the foregoing arrangement enables easyreplacement of the probe assembly 35. To remove the probe assembly, theretainer 100 is removed from the fitting 94. This allows the entireprobe assembly 35 to be withdrawn from the guide tube 74 by simplypulling the plug 98 and optical fibers 73 out of the fitting 94 and theguide tube. A replacement assembly can be installed in reverse manner,inserting the optical fibers 73 with the probes 80 and 82 at the endthereof into the guide tube 74 which guides the optical fibers and theprobes into position and finally pushing the plug 98 into the fitting94, after which the retainer 100 can be reinstalled. It is noted thereduced diameter portion of the through passage 96 in the fitting 98 issized to permit passage therethrough of the optical fibers 73 and anyprotective sheathing or tubes 116.

A gas fitting (not shown) also may be provided in the wall of thecylinder 32 to provide access for a pressure transducer and the additionof gas. Pressure data developed by the transducer also can becommunicated to the processor 39.

In operation, compression of the shock strut 10 causes the piston 30 tomove into the cylinder 32 thereby reducing the volume of the chamber 42and compressing the portion filled with gas. The compressed gas storesenergy in a manner similar to a spring. Relative telescoping movement ofthe piston 30 into the cylinder 32 pumps liquid from the generally lowerdynamic liquid chamber 56 through the orifice plate 50 into thepneumatic chamber 54 as the shock strut 10 is compressed, therebyincreasing resistance to compression through fluid amplification whilesimultaneously dissipating compression energy. As the piston 30 movesinto the cylinder 32, the metering pin 60 moves into the orifice opening52 in the orifice plate 50, effectively reducing the flow area throughthe orifice opening 52 and increasing resistance to further compression.

Part of the work expended in compressing the shock strut 10 is stored asrecoverable spring energy in the portion filled with gas whichresiliently suspends the aircraft structure 20 (FIG. 1) while taxiing onthe ground, and which also allows the piston 30 and the cylinder 32 toreturn to an extended position after the compression force is removed,such as after takeoff.

The detector 35 monitors the liquid level. As above indicated, usuallysuch monitoring is effected when the aircraft is in flight and when thestrut 10 is in a fully extended position, as when the landing gear areextended. When the liquid level drops below the threshold level 84 or 86as sensed by the detector 35, the processor 39 may initiate an audible,visual or other alarm for alerting personnel to check the liquid level.It is particularly advantageous to take liquid level measurements afterliftoff, as taxiing will usually warm the fluid to provide a moreconsistent reading regardless of cold or warm climate conditions. If thegear is retracted, the liquid (oil) usually would take longer to migrateback to its normal position where it could be checked.

Specifically, the system detects the condition of the liquid level whenthe strut 10 is in a fully extended position and the aircraft is inflight. This avoids the need to compensate for the change in liquidlevel that occurs when the strut is compressed i.e., not fully extended.Since the strut reaches the fully extended position once the aircraftleaves the ground for flight or when the strut is deployed inpreparation for landing, the probes 80 and 82 are best positioned todetermine the condition of the liquid at those times.

The processor 39 may receive a signal indicating that the aircraft hasleft the ground or that the landing gear has been deployed for landing.At that time, the processor 39 would activate the sensor unit 38 to senda beam of light through the optical fibers 73 to each probe 80/82. Inthe absence of liquid, the light will be reflected back through thereturn fiber. In the presence of liquid, no light is reflected. Thesensor unit 38 indicates whether return light was observed and this iscommunicated to the processor 39. Reflected light received from bothprobes 80 and 82 indicates the liquid level is unacceptably low.Reflected light received from only the shorter probe indicates that theliquid is low but still acceptable. Minimal reflected light wouldindicate that the strut 10 has the proper fluid level. The processor 39stores the data for retrieval by maintenance personnel and/or mayprovide an alert, such as illuminating a light in the cockpit to beobserved by flight personnel and/or in the wheel well to be observed bythe ground crew after the flight and/or before the next flight. Thoseskilled in the art will also appreciate that additional probes may beused to provide additional level readings as may be desired.

If desired, advantage can be taken from the occurrence of intermittentsignals that will occur during sensor monitoring when the oil level isnear the level of a probe. This is illustrated in FIG. 5 and thefollowing table. High probe- Low Oil Low Probe- Adversely Low OilInterpretation L1 Consistent reading of “in-Oil” Consistent reading of“in-Oil” Proper oil level L2 Intermittent readings of both Consistentreading of “in-Oil” Oil level becoming low “in-Oil” & “Out-of-Oil” L3Consistent reading of “Out-of-Oil” Consistent reading of “in-Oil” Oillevel is low. Service needs to be scheduled L4 Consistent reading of“Out-of-Oil” Intermittent readings of both Oil level is becomingadversely “in-Oil” & “Out-of-Oil” low. Service needs to be scheduledsoon L5 Consistent reading of “Out-of-Oil” Consistent reading of“Out-of-Oil” Oil level is adversely low. Service immediatelyThe intermittent readings arise from the oil level being within thesensor sensitivity range as depicted in FIG. 5.

A further illustration of fluid level sensor indications is contained inFIG. 6.

Referring now to FIGS. 7-12, another implementation of a shock strutwith level detection is illustrated.

Although the invention has been shown and described with respect tocertain illustrated embodiment, equivalent alterations and modificationswill occur to others skilled in the art upon reading and understandingthe specification and the annexed drawings. For example, although anembodiment of the invention directed to an aircraft strut is described,a shock absorber provided by the present invention may have otherapplications other than aeronautical applications. In particular regardto the various functions performed by the above described integers(components, assemblies, devices, compositions, etc.), the terms(including a reference to a “means”) used to describe such integers areintended to correspond, unless otherwise indicated, to any integer whichperforms the specified function (i.e., that is functionally equivalent),even though not structurally equivalent to the disclosed structure whichperforms the function in the herein illustrated embodiments of theinvention.

1. An aircraft shock strut, comprising a cylinder; a pistontelescopically movable within the cylinder and defining therein a sealedchamber partially filled with a liquid and partially filled with a gas;and at least one probe associated with the chamber for sensing acondition of a level of liquid in the chamber.
 2. A shock strut as setforth in claim 1, further comprising a cable that passes through a wallof the strut for connecting to the probe.
 3. A shock strut as set forthin claim 2, wherein the cable includes at least one optical fiber.
 4. Ashock strut as set forth in claim 3, wherein the probe is an opticalliquid sensing probe.
 5. A shock strut as set forth in claim 2, furthercomprising a fitting assembly that seals the cable with respect to thestrut.
 6. A shock strut as set forth in claim 5, wherein the fittingassembly includes a plug molded around the cable and a retainer forholding the plug in sealed relationship with a through passage in thestrut.
 7. A shock strut is set forth in claim 6, wherein the plug has anannular groove for receiving an O-ring seal.
 8. A shock strut as setforth in claim 6, wherein the cable includes at least one optical fiberand plug is molded directly to the optical fiber to effect a seal aroundthe optical fiber.
 9. A shock strut as set forth in claim 6, wherein thecable includes a plurality of optical fibers that have transverselyspaced apart, coextending portions thereof each surrounded in sealedrelationship by the plug that has been molded thereto.
 10. A shock strutas set forth in claim 2, wherein the probe and cable are assembledtogether as a unit, and wherein a guide tube is mounted within thechamber, the unit at least partially extending through and being locatedby the guide tube.
 11. A shock strut as set forth in claim 10, whereinthe unit is removable as a unitary piece from the strut.
 12. A shockstrut as set forth in claim 1, wherein the at least one probe includes aplurality of probes spaced apart along a longitudinal axis of the strut.13. A shock strut as set forth in claim 1, wherein the probe is a liquidlevel sensing fiber optic probe.
 14. A shock strut as set forth in claim1, wherein the at least one probe includes two probes, a first one ofwhich detects a condition of a first liquid level and a second one ofwhich detects a condition of a second liquid level.
 15. A systemcomprising the aircraft shock strut as set forth in claim 1, furthercomprising a processor in communication with the probe for processing asignal from the probe related to the level of liquid in the chamber. 16.A system as set forth in claim 15, wherein probe is a level sensingoptical probe, and further comprising a sensor unit external to thechamber and connected by an optical cable to the probe within thechamber, the sensor unit functioning to transmit light to the probe andreceive reflected light from the probe via the optical cable, andwherein the sensing unit is connected to the processor.
 17. A system asset forth in claim 1, wherein probe is a level sensing optical probe,and further comprising a sensor unit external to the chamber andconnected by an optical cable to the probe within the chamber, thesensor unit functioning to transmit light to the probe and receivereflected light from the probe via the optical cable.
 18. A method ofmonitoring a liquid level in an aircraft shock strut comprising thesteps of: receiving a signal related to the liquid level from the atleast one probe of a shock strut according to claim 1; and processingthe signal to determine a characteristic of the liquid level.
 19. Amethod of monitoring as set forth in claim 18, wherein the liquid levelis sensed in a fully extended position of the strut.
 20. A shockabsorber, comprising: comprising a cylinder; a piston telescopicallymovable within the cylinder and defining therein a sealed chamberpartially filled with a liquid and partially filled with a gas; and atleast one probe associated with the chamber for sensing a condition of alevel of liquid in the chamber.
 21. A shock absorber as set forth inclaim 15, wherein the at least one probe includes at least one fiberoptic probe.
 22. A shock absorber as set forth in claim 16, wherein thedistal end of the probe includes a retro-reflective prism.